linux-kernel - RE: [PATCH hyperv-next v6 05/17] arch/x86: mshyperv: Trap on access for some synthetic MSRs

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Message-ID:
 <SN6PR02MB41571BD37714C5F0AB770CB5D4E3A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Mon, 6 Oct 2025 16:55:20 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Roman Kisel <romank@...ux.microsoft.com>, "arnd@...db.de" <arnd@...db.de>,
	"bp@...en8.de" <bp@...en8.de>, "corbet@....net" <corbet@....net>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"decui@...rosoft.com" <decui@...rosoft.com>, "haiyangz@...rosoft.com"
	<haiyangz@...rosoft.com>, "hpa@...or.com" <hpa@...or.com>,
	"kys@...rosoft.com" <kys@...rosoft.com>, "mikelley@...rosoft.com"
	<mikelley@...rosoft.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "Tianyu.Lan@...rosoft.com"
	<Tianyu.Lan@...rosoft.com>, "wei.liu@...nel.org" <wei.liu@...nel.org>,
	"x86@...nel.org" <x86@...nel.org>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-arch@...r.kernel.org"
	<linux-arch@...r.kernel.org>
CC: "benhill@...rosoft.com" <benhill@...rosoft.com>, "bperkins@...rosoft.com"
	<bperkins@...rosoft.com>, "sunilmut@...rosoft.com" <sunilmut@...rosoft.com>
Subject: RE: [PATCH hyperv-next v6 05/17] arch/x86: mshyperv: Trap on access
 for some synthetic MSRs

From: Roman Kisel <romank@...ux.microsoft.com> Sent: Friday, October 3, 2025 3:27 PM
> 
> hv_set_non_nested_msr() has special handling for SINT MSRs
> when a paravisor is present. In addition to updating the MSR on the
> host, the mirror MSR in the paravisor is updated, including with the
> proxy bit. But with Confidential VMBus, the proxy bit must not be
> used, so add a special case to skip it.
> 
> Signed-off-by: Roman Kisel <romank@...ux.microsoft.com>
> Reviewed-by: Alok Tiwari <alok.a.tiwari@...cle.com>
> Reviewed-by: Tianyu Lan <tiala@...rosoft.com>
> ---
>  arch/x86/kernel/cpu/mshyperv.c | 29 +++++++++++++++++++++++++----
>  drivers/hv/hv_common.c         |  5 +++++
>  include/asm-generic/mshyperv.h |  1 +
>  3 files changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index af5a3bbbca9f..b410b930938a 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -28,6 +28,7 @@
>  #include <asm/apic.h>
>  #include <asm/timer.h>
>  #include <asm/reboot.h>
> +#include <asm/msr.h>
>  #include <asm/nmi.h>
>  #include <clocksource/hyperv_timer.h>
>  #include <asm/msr.h>
> @@ -38,6 +39,12 @@
>  bool hv_nested;
>  struct ms_hyperv_info ms_hyperv;
> 
> +/*
> + * When running with the paravisor, controls proxying the synthetic interrupts
> + * from the host
> + */
> +static bool hv_para_sint_proxy;

This needs to move down a few lines and be under the #if IS_ENABLED(CONFIG_HYPERV)
in order to eliminate the "unused variable" warning reported by the kernel test robot.

> +
>  /* Used in modules via hv_do_hypercall(): see arch/x86/include/asm/mshyperv.h */
>  bool hyperv_paravisor_present __ro_after_init;
>  EXPORT_SYMBOL_GPL(hyperv_paravisor_present);
> @@ -79,17 +86,31 @@ EXPORT_SYMBOL_GPL(hv_get_non_nested_msr);
>  void hv_set_non_nested_msr(unsigned int reg, u64 value)
>  {
>  	if (hv_is_synic_msr(reg) && ms_hyperv.paravisor_present) {
> +		/* The hypervisor will get the intercept. */
>  		hv_ivm_msr_write(reg, value);
> 
> -		/* Write proxy bit via wrmsl instruction */
> -		if (hv_is_sint_msr(reg))
> -			wrmsrq(reg, value | 1 << 20);
> +		/* Using wrmsrq so the following goes to the paravisor. */
> +		if (hv_is_sint_msr(reg)) {
> +			union hv_synic_sint sint = { .as_uint64 = value };
> +
> +			sint.proxy = hv_para_sint_proxy;
> +			native_wrmsrq(reg, sint.as_uint64);
> +		}
>  	} else {
> -		wrmsrq(reg, value);
> +		native_wrmsrq(reg, value);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(hv_set_non_nested_msr);
> 
> +/*
> + * Enable or disable proxying synthetic interrupts
> + * to the paravisor.
> + */
> +void hv_para_set_sint_proxy(bool enable)
> +{
> +	hv_para_sint_proxy = enable;
> +}
> +
>  /*
>   * Get the SynIC register value from the paravisor.
>   */
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index 8756ca834546..1a5c7a358971 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -716,6 +716,11 @@ u64 __weak hv_tdx_hypercall(u64 control, u64 param1, u64
> param2)
>  }
>  EXPORT_SYMBOL_GPL(hv_tdx_hypercall);
> 
> +void __weak hv_para_set_sint_proxy(bool enable)
> +{
> +}
> +EXPORT_SYMBOL_GPL(hv_para_set_sint_proxy);
> +
>  u64 __weak hv_para_get_synic_register(unsigned int reg)
>  {
>  	return ~0ULL;
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index c010059f1518..3955ba6d60b8 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -298,6 +298,7 @@ bool hv_is_isolation_supported(void);
>  bool hv_isolation_type_snp(void);
>  u64 hv_ghcb_hypercall(u64 control, void *input, void *output, u32 input_size);
>  u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2);
> +void hv_para_set_sint_proxy(bool enable);
>  u64 hv_para_get_synic_register(unsigned int reg);
>  void hv_para_set_synic_register(unsigned int reg, u64 val);
>  void hyperv_cleanup(void);
> --
> 2.43.0
>